Sulphur BPT diagram for unattenuated line emission with added random noise. Colours are the same as in Fig. 6. On the lower panel, noise is higher than the real attenuation percentage (around 80% ± 10%, see Table 2). On the upper panel, noise is lower than the calculated attenuation (around 20% ± 10%). The upper panel’s circles can be well separated during the whole time evolution (from left to right).